Circuit cards, for example, conduction-cooled circuit card assemblies (CCAs) complying with international standards such as IEEE Std. 1101.2-1992, ANSI-VITA 30.1 and VITA 48.2, can be mounted in an enclosure such as a card cage, chassis, rack, package or case, and can also be easily removed from the enclosure for replacement or repair.
A typical card cage enclosure includes a plurality of card slots, each card slot defined by a pair of card guides positioned along first and second opposed sides of a card cage, for example, at a top and bottom of the card cage, or at a left side and right side of the card cage. This configuration permits the circuit card to be properly seated in the card cage to ensure electrical and thermal registration of the circuit card in the card cage.
During operation, electronic components on the circuit card can generate a significant amount of thermal energy that causes the temperature inside the card cage to increase, in particular, in configurations where a plurality of circuit cards densely populates the card cage. However, an excessive increase in temperature in the card cage can result in damage to circuit cards residing in the card cage, or more specifically, to electronic components on the circuit cards.
According to some conventional approaches, the temperature inside the card cage can be reduced by natural convection, for example, using exterior fins or slots in the card cage to remove heat from the card cage to an ambient environment, and to draw cooler air into the card cage.
In other approaches, the temperature can be reduced by forced convection, for example, using fans, or using pipes carrying cooling liquids through channels integrated into the walls of the card cage.
In other approaches, heat generated by electronic components on a circuit card can be at least partially removed by conductive cooling, i.e., dissipation or removal of heat from the circuit card to the surrounding card cage chassis via a conduction frame, which conducts heat away from the electronic components on the circuit card to the card cage chassis, for transfer to the ambient environment.
FIG. 1 is an illustrative view of a conventional arrangement of a CCA module 120 mounted or inserted in a conduction-cooled card cage 110. FIG. 2A is a front view of the conventional arrangement of the CCA module 120 mounted in the conduction-cooled card cage 110 of FIG. 1, illustrating a dry-contact interface A between the CCA module 120 and the card cage 110. FIG. 2B is a graph illustrating a typical temperature profile across the dry-contact interface A illustrated in FIGS. 1 and 2A.
The CCA module 120 includes a conduction frame 150 that is secured to a card guide 111 integrated into a sidewall of the card cage 110 by two wedge clamps 130, also referred to as wedge locks.
The conduction frame 150 includes a conduction plate 151 coupled to the bottom surface of a substrate 140 of the CCA module 120. The conduction plate 151 is thermally coupled to electronic components 121 that populate a top surface of the substrate 140 by a plurality of thermally conductive vias 153 that are formed through the substrate 140.
Accordingly, heat that is generated by the electronic components 121 on the CCA module 120 can be removed by one or more conduction paths formed between the electronic component 121 and the card cage 110. In particular, a first conduction flow path, indicated by a dotted line (i) in FIG. 2A, can be formed between electronic component 121, through a thermally conductive via 153 in the conduction frame 140, and a first thermal interface A, referred to as a “dry-contact interface,” or bare junction thermal interface, between the conduction plate 151 and the card guide 111. Although a dry-contact interface can refer to one metal surface that is directly on another metal surface, such as a metal-to-metal interface, a dry-contact interface can also include intervening elements, such as gaskets, adhesives, etc. positioned between the two surfaces.
The thermal resistance at thermal interface A depends largely in part on the amount of force exerted by the wedge clamps 130 against the surface of the card guide 111. A greater force applied by the wedge clamp 130 against the upper surface of the card guide 111 can decrease the thermal resistance at thermal interface A, resulting in improved heat flow along the conduction flow path, but such contact force alone is limited in its ability to reduce the thermal resistance. On the other hand, a lesser force applied by the wedge clamp 130 to the upper surface of the card guide 111 can lead to an undesirable higher thermal resistance at thermal interface A.
In high-power conduction-cooled applications, a high thermal resistance at thermal interface A can lead to significant thermal performance problems. For example, a typical thermal interface exhibiting a thermal resistance of 0.2° C./watt to 0.4° C./watt, and a 150 Watt CCA with heat flux equally distributed on two thermal interfaces, also referred to as wedge clamp interfaces, can experience a 15° C. to 30° C. temperature rise across each wedge clamp interface, i.e., location “A” in FIGS. 2A and 2B. As shown in FIG. 2B, this can result in the edge of CCA module 120 and card cage 110 having a substantial temperature difference ΔT, for example, the edge of the CCA module 120 having a temperature that is 15° C. to 30° C. higher than the surface of the card cage 110. This can result in the CCA module 120 being unusable in many applications because the CCA module 120 can overheat in a typical high temperature ambient environment of 70° C. without sufficient cooling.
In addition to the abovementioned first conduction flow path, a second conduction flow path, shown by dashed line (ii) in FIG. 2A, can be formed between electronic component 121 and thermal interface B between wedge clamp 130 and card guide 111. This second conduction flow path (ii) is not as effective for removing heat as the abovementioned first conduction flow path (i), since the main body of the wedge clamp 130 is not in good thermal contact with the top portion 131 of the wedge clamp 130, which can exhibit high thermal resistance when the top portion 131 of the wedge clamp 130 is separated from the main body of the wedge clamp 130 when the wedge clamp 130 is expanded to hold the CCA module 120 firmly in place against the card guide 111, and since the thermal interface B between the top portion 131 and the top surface of the card guide 111 can also exhibit a high thermal resistance.